<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>按钮...</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    body {
      width: 100vw;
      height: 100vh;
      display: grid;
      grid-template: 1fr/1fr;
    }
    button {
      width: 200px;
      height:50px;
      background: #ecf0f1;
      color: #34495e;
      border:1px solid #ddd;
      justify-self: center;
      align-self: center;
    }
    button::after {
      content: '';
      width: 3px;
      height: 3px;
      display: inline-block;
      /* box-shadow: 3px 0 0 currentColor, 9px 0 0 currentColor, 15px 0 0 currentColor; */
      animation-name: loading;
      animation-duration: 1s;
      animation-iteration-count: 1;
      /* animation-fill-mode: forwards;  */
    }
    @keyframes loading {
      from {
        box-shadow: none;
      }
      30% {
        box-shadow: 3px 0 0 currentColor;
      }
      60% {
        box-shadow: 3px 0 0 currentColor, 9px 0 0 currentColor;
      }
      90% {
        box-shadow: 3px 0 0 currentColor, 9px 0 0 currentColor, 15px 0 0 currentColor;
      }
    }
  </style>
</head>
<body>
  <button>提交</button>
</body>
</html>